We have partially characterized an isolate of simian varicella- zoster virus (VZV), which we have named HHV. The genome of HHV was found to be identical to the Delta VZV, a well studied simian VZV, in 15 of 17 restriction sites, suggesting that the genomic nucleotide base sequence of these 2 VZV's are closely related. HHV, Delta and human VZV also showed antigenic cross-reactivity when compared by ELISA. Reciprocal tests employing HHV or Delta VZV virus and type specific antisera gave virtually identical results regardless of which virus or antiserum was used. However, antiserum to human VZV virus has lower antibody titers when tested against both HHV and Delta VZV then against homeotypic virus. Although HHV and Delta VZV gave similar results by ELISA, differences were found by immunoprecipitation. Antiserum to Delta VZV failed to immunoprecipitate an 89K glycoprotein from HHV VZV. Furthermore, reciprocal neutralization tests showed that antiserum to HHV or Delta VZV gave type specific results, the homeotypic virus being neutralized to far higher titer than the heterotypic virus. These results suggest that HHV is a new strain of simian VZV.